The present invention relates to radio communication systems and in particular to the detection of disturbances in such systems in which Carrier over Interference (C/I) ratio is used to measure disturbances.
In, for example, most Frequency Division Multiple Access (FDMA) systems the received signal on each channel is filtered out using an Intermediate Frequency (IF) filter. Variations in the amplitude of the received signal are then used to detect possible disturbance. Ideally, the IF filter should let the entire signal through in the appropriate frequency range of the filter and filter out all signals having frequencies outside of this range. With an ideal IF filter, therefore, an undisturbed signal would have a uniform amplitude for all frequencies within the frequency range of the channel. Since the IF filters used are not ideal, the signal is attenuated to a varying degree, depending on the frequency, so that the signal strength varies. Since the frequency of the signal is never entirely stable, the amplitude of the signal varies. The frequency of the signal varies, for example, with the amplitude of the modulated signal.
Such fluctuations in the amplitude of the signal are often mistaken for disturbances from another radio transmitter transmitting on the same frequency. The variations in frequency differ between different types of systems.
The same type of problems may occur in any type of radio communications network, where other type of transmitters than base stations may be used for communication with the mobile terminals.
Various measures can be taken to compensate for disturbances. Often a mobile telephone is ordered to change its frequency when a disturbance is detected. If there is another base station in the vicinity, the mobile telephone may be ordered to connect to this other base station instead of the one to which it is connected. The mobile telephone may also be ordered to increase its output power, to increase the C/I ratio. In extreme cases a radio transmitter may be ordered to disconnect from the base station.
These measures may be effective if taken when there is really a disturbance. If a disturbance is erroneously detected, they are, of course, useless. In prior art mobile telephone systems a mobile telephone may be ordered to change channels again and again, because a disturbance is erroneously detected.
Attempts to solve this problem include building better IF-filters. Building ideal filters is expensive, or even impossible.
It has also been attempted to compensate in the received signal for the imperfection of the filter. This requires very high processor capacity since each individual filter has different characteristics. Delay and beat must be taken into account, which is complicated.
It is an object of the present invention to provide a method and an apparatus for the reliable measurements of the disturbances to a received signal.
This object is achieved according to the invention by a C/I measuring apparatus for a signal having a fluctuating carrier frequency, comprising a filter for filtering the signal and means for splitting the signal into one part (RSS) representing the signal strength and one baseband part (BB) representing the momentary carrier frequency deviation, comprising means for measuring the RSS signal unaffected by the frequency dependent attenuation in the IF filter.
The object is also achieved according to the invention by a method of measuring the C/I of a received signal comprising the following steps:
Filtering the signal is filtered using an IF filter to obtain one signal channel. feeding the signal to the FM detector and the RSSI unit,
measuring the signal strength of the received signal at a selected, substantially constant, carrier frequency.
According to a preferred embodiment the C/I measuring apparatus comprises means for determining at least one point in time when the BB signal is substantially equal to a constant carrier frequency deviation.
By measuring the received signal at points in time where the filter attenuation is substantially equal, the measurements become more reliable without the need for compensation.
In a first embodiment the C/I measuring apparatus comprises a level detecting unit for monitoring the BB signal and a sampling unit, the level detecting unit controlling the sampling unit in such a way that when the amplitude of the BB signal is equal to the level specified in the level detecting unit, the sampling unit samples the RSS signal.
In the first embodiment of the invention the method comprises the step of determining at least one point in time at which the BB signal is substantially equal to a constant carrier frequency deviation and measuring the RSS signal at this point in time.
This first embodiment can be implemented by software change only, to a prior art receiver unit.
In a second embodiment, the C/I measuring apparatus comprises means for determining at least one point in time at which the BB signal is substantially equal to the level specified in the level detecting unit and measuring the RSS signal at this point in time.
In the second embodiment the method comprises the steps of
dividing the BB into measurement sequences;
taking at least one sample in each measurement sequence the BB signal;
selecting for each measurement sequence at least one sample nearest to the zero level; and
sampling the RSS signal at the point or points in time corresponding to the at least one selected sample of the BB signal.
This is a hardware-based embodiment, which requires less processing power than the software-based embodiment, since the sampling is performed by hardware units.
In the second embodiment the C/I measuring device may also comprise means for
dividing the BB signal into measurement sequences;
taking at least two samples in each measurement sequence of the BB signal; and
interpolating between at least two samples in each measurement sequence to obtain at least one point in which the carrier frequency deviation is constant and hence the IF filter attenuation is constant, of the BB signal;
sampling the RSS signal at the point or points in time corresponding to the at least one calculation point of the BB signal.
This enables an more reliable approximation of the value of the signal at the exactly right point in time.
Preferably, the carrier frequency is selected so that the attenuation of the filter is substantially stable for frequencies near this carrier frequency.
This reduces the effects of small fluctuations of the frequency.
The selected carrier frequency may be substantially the centre frequency of the filter.